1. Field of the Invention
The present invention relates to a method of frequency tuning a piezoelectric device and an apparatus for the implementation of the method.
2. Description of the Related Art
By piezoelectric device is meant a plate made of piezoelectric material comprising at least one resonating function formed by two electrodes facing each other, each on a different principal face of the plate. The plate is fixed to a base which comprises electrical connections connected to the electrodes.
A piezoelectric device may combine together a plurality of resonating functions on the same plate. When the electrodes of a plurality of resonating functions are very close to each other, the functions are mechanically coupled. The coupled resonating functions may have a common electrode. The electrodes are constituted by a metal layer deposited on the plate made of piezoelectric material.
The value of the resonant frequency of a resonating function is determined in the first instance by the thickness of the plate made of piezoelectric material, but also by the thickness of the metal of the electrodes. In the case where the resonating functions are mechanically coupled, the value of the coupling depends on the proximity of the electrodes from the two resonating functions, but also on the thickness of the metal of the electrodes.
Several methods are currently employed to tune the frequency of resonating functions. They divide into two categories. Metal addition methods consist in metalizing a plate initially lacking in metallization or in the overlaying of an already partially metallized plate. Metallization removal methods consist thinning down at least one predeposited electrode.
Among the metal addition methods, mention may be made of the electrolytic deposition of gold on at least one predeposited electrode. During this method, the electrode is electrically connected up to a controlled current source, of which it constitutes the cathode. The plate made of piezoelectric material is dipped into a bath of gold salts. Application of the current brings about the deposition of gold on the electrode. The control of the current, the temperature, the concentration, the pH and the immersion time enable the frequency tuning to be performed. This method enables a plurality of resonating functions to be frequency tuned. All that is required is to connect up at least one electrode for each resonating function to the current source. This method is difficult to automate because the measurement of the frequency of the resonating function can only be carried out while the plate made of piezoelectric material is immersed.
Mention may also be made of the vacuum deposition of metal. The plate made of piezoelectric material is introduced into a chamber in which a high vacuum is maintained and in which there are two metallization sources facing each other. The plate is already fixed to its base which comprises electrical connections, but the electrodes have not been deposited.
Masks are interposed between the sources and the plate. The masks have apertures corresponding the dimensions of the electrodes to be deposited. The masks are not in contact with the plates on account of the fixing and connection devices.
Movable shutters enable the apertures of the masks to be occulted as required. The frequency of each resonating function is measured during the metallization, which is deposited only at the locations left exposed by the apertures of the masks. As soon as the frequency of a resonating function has reached its target value, the apertures of the masks corresponding to the electrodes of this function are occulted by interposing the shutters. The main drawback of this method resides in the fact that the contour of the electrodes lacks precision since the masks are not in contact with the plate.
Among the removal methods, mention may be made of the chemical etching of the electrodes. In this method, the electrodes are predeposited on the plate made of piezoelectric material. At least one electrode is subjected to the action of a chemical agent etching the metal of the electrode. This action is applied by virtue of a pad imbibed with chemical agent. Discrimination of the electrodes is obtained by selective application of the pad. This method does not enable the frequency of the resonating functions to be measured during application of the pad. This method is limited toward high frequencies. In this case, the electrodes are very small (of the order of one hundredth of a square millimeter) and discrimination of one electrode among a plurality is not possible with the pad.
The method according to the invention is to overcome the drawbacks of the known methods.